1. Field of The Invention
The present invention relates to an optical pick-up apparatus, which is used to record information into an optical disk and to reproduce the recorded information from the same.
2. Description of The Prior Art
An optical pick-up apparatus associated with an optical disk device utilizes a laser beam for record and reproduction of information. In order to realize the reduction of size and weight in such an optical pick-up apparatus, a plurality of holographic optical elements have been conventionally used. And, there is known a method of reflecting a laser beam several times in a light guide. For instance, the Japanese Unexamined Patent Applications Nos. 146444/1987 and 155529/1989 disclose a long and thin light guide made of transparent substance. In this light guide, a going laser beam is reflected several times and reaches an optical disk passing through a holographic optical element. The optical disk memorizes information on the surface thereof in the form of unit of spots or depressions. A returning laser beam reflected at the recording surface of the optical disk is again reflected several times in the light guide but is guided via a different path to an photo detector. The Japanese Unexamined Patent Application No. 20737/1988 discloses two holographic optical elements. One converges a going laser beam onto an optical disk and the other converges a returning laser beam reflected at the optical disk to a photo detector.
Furthermore, the Japanese Unexamined Patent Applications. Nos. 81335/1990 and 220145/1989 disclose holographic optical elements integrally assembled with objective lenses used for causing diffraction in a returning beam after being reflected at an optical disk.
Hereinafter, above-introduced conventional optical pick-up apparatus equipped with holographic optical elements will be explained in more detail with reference to FIGS. 22 to 25. FIG. 22 shows an optical pick-up apparatus utilizing a plurality of reflections of a laser beam, as represented by the Japanese Unexamined Patent Applications Nos. 146444/1987 and 155529/1989. FIG. 23 shows an optical pick-up apparatus including a pair of holographic optical elements, one holographic optical element converging a going laser beam and the other holographic optical element causing diffraction of a returning laser beam, which is similar to that disclosed in the Japanese Unexamined Patent Application No. 20737/1988. FIG. 24 shows an optical pick-up apparatus using an objective lens on the spherical surface of which a holographic optical element is integrally or directly formed to cause diffraction of a laser beam, the same type as that disclosed by the Japanese Unexamined Patent Application No. 81335/1990. FIG. 25 shows an optical pick-up apparatus using a complex objective lens whose body is split into two by a plane normal to an axis thereof. A holographic optical element for diffraction is sandwiched therebetween, as is disclosed in the Japanese Unexamined Patent Application No. 220145/1989.
In FIG. 22, a reference numeral 501 represents a laser diode emitting a laser beam. A reference numeral 504 represents a holographic optical element converging the laser beam emitted from the laser diode 501 onto an optical disk 503 whose surface is formed with the unit of spots memorizing information being sensed by the laser beam. A reference numeral 502 represents a light guide which is made of a transparent substance such as fused silica and causes a laser beam to reflect repeatedly at inside, upper and lower, surfaces thereof. A reference numeral 505 represents a holographic optical element of reflection type which reflects a returning laser beam having once reached and been reflected at the surface of the optical disk 503 and also diffracts this returning laser beam toward a photo detector 506.
In FIG. 23, a reference numeral 507 represents a holographic optical element of transmission type which transmits a returning laser beam having been reflected at the surface of the optical disk 503 and also diffracts this returning laser beam toward the photo detector 506. The laser diode 501, optical disk 503, holographic optical element 504, and photo detector 506 are substantially the same as those explained with reference to FIG. 22.
In FIG. 24, a reference numeral 508 represents an objective lens converging a laser beam emitted from the laser diode 501 onto the optical disk 503. A reference numeral 509 represents a holographic optical element of transmission type which transmits a returning laser beam having been reflected at the surface of the optical disk 503 and also diffracts this returning laser beam toward the photo detector 506. The laser diode 501, the optical disk 503, and the photo detector 506 are substantially the same as those explained with reference to FIG. 22.
In FIG. 25, a reference numeral 510 represents a complex objective lens whose body is split by a plane normal to an axis thereof into two, upper and lower, half bodies. This complex objective lens 510 converges a laser beam emitted from the laser diode 501 onto the surface of the optical disk 503. A reference numeral 511 represents a holographic optical element of transmission type which transmits a returning laser beam having been reflected at the surface of the optical disk 503 and also diffracts this returning laser beam toward the photo detector 506. This holographic optical element 511 is sandwiched by and integrally fabricated with the paired half bodies of the complex objective lens 510. The laser diode 501, the optical disk 503, and the photo detector 506 are substantially the same as those explained with reference to FIG. 22.
Operations on above introduced conventional optical pick-up apparatus will be explained below.
In FIG. 22, a laser beam is emitted from the laser diode 501 and, then, reflected plural times at inside, upper and lower, surfaces of the light guide 502 so as to reach the holographic optical element 504. The holographic optical element 504 then converges the laser beam thus guided through the light guide 502 onto the surface of the optical disk 503. The laser beam is reflected at the surface, i.e. a recording surface, of the optical disk 503 and returns as a beam including information read out from the optical disk 503. The returning beam passes through the holographic optical element 504 again and, in turn, reaches the holographic optical element 505 of reflection type. This holographic optical element 505 not only reflects the returning laser beam but diffracts it toward the photo detector 506. The photo detector 506 receives the returning laser beam and detects focusing error and tracking error, as well as the read-out information.
The optical pick-up apparatus shown in FIGS. 23, 24, and 25 operate in the same manner as that shown in FIG. 22. A laser beam emitted from the laser diode 501 is converged onto the recording surface of the optical disk 503 passing through the holographic optical element 504, the objective lens 508, or the split-type complex objective lens 510, respectively. After having been reflected, the returning beam comprising information read out from the optical disk 503 passes through the holographic optical elements 507, 509, or 511 respectively and is diffracted toward the photo detector 506. The photo detector 506 receives the returning laser beam and detects focusing error and tracking error, as well as the read-out information.
These prior art constructions of the optical pick-up apparatus are, however, disadvantageous in complicateness of adjusting positional relationship between individual optical components. One reason of requiring complicated positional adjustment of the optical components is that the converging arrangement and the diffracting arrangement are independent from each other. In more detail, the converging arrangement converging a laser beam is constituted by the holographic optical element 504, the objective lens 508, or the split-type objective lens 510. On the other hand, the diffracting arrangement diffracting the laser beam is constituted by the reflection-type holographic optical element 505 or the transmission-type holographic optical element 507, 509, 511. These converging arrangement and the diffracting arrangement are independently mounted on the light guide 502 or installed into a casing. Therefore, it was inevitable to take a long time in an installation or fabrication of the pick-up apparatus because of not only positional adjustment of individual optical components but mutual adjustment of positional relationship between the converging construction and the diffracting construction. This complicateness in the manufacturing process results in increase of production cost.
Furthermore, in case of the optical pick-up apparatus shown in FIG. 24, the constructional requirement of forming the transmission-type holographic optical element 509 directly on the objective lens 508 further increases the complexity. This will be easily understood from the spherical surface of the objective lens 508 which makes the formation of the transmission-type holographic optical element 509 thereon difficult. Still further, in case of the optical pick-up apparatus shown in FIG. 25, the structure of sandwiching the transmission-type holographic optical lens 511 between the split half bodies of the complex objective lenses 510 is not only time-consuming in its assembling but tends to cause unacceptable deterioration in lens property.